Laminated electrode for a cochlear implant

ABSTRACT

An implantable cochlear implant electrode array ( 30 ) that can adopt a first, preferably straight, configuration selected to allow the array to be inserted into the cochlea and at least a second, preferably, spirally curved, configuration wherein the electrode array ( 30 ) is adapted to apply tissue stimulation. The electrode array ( 30 ) comprises an elongate carrier ( 31 ) having a proximal end ( 37 ), a distal end ( 34 ), and a plurality of electrodes supported by the carrier ( 31 ) at respective spaced locations thereon, the carrier ( 31 ) is formed, preferably moulded, to preferentially adopt the second configuration or another configuration different to said first configuration. The outer layer ( 33 ) is releasably connected to the elongate carrier ( 31 ) by an adhesive layer ( 32 ) and is formed so as to bias the carrier ( 31 ) into the first configuration when connected thereto. A method of forming the electrode array ( 30 ) is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Phase Patent Application of InternationalApplication Number PCT/AU02/00279, filed on Mar. 12, 2002, which claimspriority of Australian Patent Application Number PR 4080, filed on Mar.29, 2001.

FIELD OF THE INVENTION

The present invention relates to an implantable device and, inparticular, to an implantable cochlear electrode assembly. A method ofmanufacturing such a device is also described.

BACKGROUND OF THE INVENTION

Hearing loss, which may be due to many different causes, is generally oftwo types, conductive and sensorineural. Of these, conductive hearingloss occurs where the normal mechanical pathways for sound to reach thehair cells in the cochlea are impeded, for example, by damage to theossicies. Conductive hearing loss may often be helped by use ofconventional hearing aids, which amplify sound so that acousticinformation does reach the cochlea and the hair cells.

In many people who are profoundly deaf, however, the reason for theirdeafness is sensorineural hearing loss. This type of hearing loss is dueto the absence of, or destruction of, the hair cells in the cochleawhich transduce acoustic signals into nerve impulses. These people arethus unable to derive suitable benefit from conventional hearing aidsystems, no matter how loud the acoustic stimulus is made, because thereis damage to or absence of the mechanism for nerve impulses to begenerated from sound in the normal manner.

It is for this purpose that cochlear implant systems have beendeveloped. Such systems bypass the hair cells in the cochlea anddirectly deliver electrical stimulation to the auditory nerve fibres,thereby allowing the brain to perceive a hearing sensation resemblingthe natural hearing sensation normally delivered to the auditory nerve.U.S. Pat. No. 4,532,930, the contents of which are incorporated hereinby reference, provides a description of one type of traditional cochlearimplant system.

Typically, cochlear implant systems have consisted of essentially twocomponents, an external component commonly referred to as a processorunit and an internal implanted component commonly referred to as areceiver/stimulator unit. Traditionally, both of these components havecooperated together to provide the sound sensation to a user.

The external component has traditionally consisted of a microphone fordetecting sounds, such as speech and environmental sounds, a speechprocessor that converts speech into a coded signal, a power source suchas a battery, and an external transmitter coil.

The coded signal output by the speech processor is transmittedtranscutaneously to the implanted receiver/stimulator unit situatedwithin a recess of the temporal bone of the user. This transcutaneoustransmission occurs via the external transmitter coil which ispositioned to communicate with an implanted receiver coil provided withthe receiver/stimulator unit. This communication serves two essentialpurposes, firstly to transcutaneously transmit the coded sound signaland secondly to provide power to the implanted receiver/stimulator unit.Conventionally, this link has been in the form of a radio frequency (RF)link, but other such links have been proposed and implemented withvarying degrees of success.

The implanted receiver/stimulator unit traditionally includes a receivercoil that receives the coded signal and power from the externalprocessor component, and a stimulator that processes the coded signaland outputs a stimulation signal to an intracochlea electrode assemblywhich applies the electrical stimulation directly to the auditory nerveproducing a hearing sensation corresponding to the original detectedsound.

Traditionally, at least the speech processor of the external componentryhas been carried on the body of the user, such as in a pocket of theuser's clothing, a belt pouch or in a harness, while the microphone hasbeen mounted on a clip mounted behind the ear or on the lapel of theuser.

More recently, due in the main to improvements in technology, thephysical dimensions of the sound processor have been able to be reducedallowing for the external componentry to be housed in a small unitcapable of being worn behind the ear of the user. This unit allows themicrophone, power unit and the sound processor to be housed in a singleunit capable of being discretely worn behind the ear, with the externaltransmitter coil still positioned on the side of the user's head toallow for the transmission of the coded sound signal from the soundprocessor and power to the implanted stimulator unit.

It is known in the art that the cochlea is tonotopically mapped. Inother words, the cochlea can be partitioned into regions, with eachregion being responsive to signals in a particular frequency range. Thisproperty of the cochlea is exploited by providing the electrode assemblywith an array of electrodes, each electrode being arranged andconstructed to deliver a stimulating signal within a preselectedfrequency range to the appropriate cochlea region. The electricalcurrents and electric fields from each electrode stimulate the nervesdisposed on the modiolus of the cochlea.

It has been found that in order for these electrodes to be effective,the magnitude of the currents flowing from these electrodes and theintensity of the corresponding electric fields, are a function of thedistance between the electrodes and the modiolus. If this distance isrelatively great, the threshold current magnitude must be larger than ifthe distance is relatively small. Moreover, the current from eachelectrode may flow in all directions, and the electrical fieldscorresponding to adjacent electrodes may overlap, thereby causingcross-electrode interference. In order to reduce the thresholdstimulation amplitude and to eliminate cross-electrode interference, itis advisable to keep the distance between the electrode array and themodiolus as small as possible. This is best accomplished by providingthe electrode array in a shape which generally follows the shape of themodiolus. Also, this way the delivery of the electrical stimulation tothe auditory nerve is most effective as the electrode contacts are asclose to the auditory nerves that are particularly responsive toselected pitches of sound waves.

In order to achieve this electrode array position close to the insidewall of the cochlea, the electrode assembly can be designed such that itassumes this position upon or immediately following insertion into thecochlea. This is a challenge as the assembly needs to be shaped suchthat it assumes a curved shape to conform with the shape of the modiolusand must also be shaped such that the insertion process causes minimaltrauma to the sensitive structures of the cochlea. In this regard, ithas been found to be desirable that the electrode assembly be generallystraight during the insertion procedure.

Several procedures have been adopted to provide an electrode assemblythat is relatively straight during initial insertion while adopting acurved configuration following insertion in the cochlea. In one case, aplatinum wire stylet is used to hold a pre-curved electrode assembly ina generally straight configuration up until insertion. The platinum wireis inserted into lumen or channel located in the pre-curved electrodeassembly with such lumen/channel allowing a passageway to accommodatethe stylet. During or immediately following insertion, the platinumstylet is withdrawn allowing the assembly to return to its pre-curvedconfiguration.

A number of methods of positioning the electrode assembly closer to themodiolus have also been developed. A number of these methods involve theuse of space-filling positioners which are inserted in the cochlea andact to essentially fill the space inside the cochlea and behind theinserted electrode array, thereby forcing the electrode array intoposition close to the modiolus. Such methods are however very invasiveand have a greater potential to cause damage to the existing sensitivestructures of the cochlea. Also such techniques prevent the user fromutilising any residual hearing that may be present and severely impacton the natural hydrodynamic behaviour of the cochlea.

The present invention is directed to an electrode assembly that canpreferably be inserted more deeply into the cochlea whilst alsopreferably reducing the degree of trauma to the sensitive structureswithin the cochlea.

Any discussion of documents, acts, materials, devices, articles or thelike which has been included in the present specification is solely forthe purpose of providing a context for the present invention. It is notto be taken as an admission that any or all of these matters form partof the prior art base or were common general knowledge in the fieldrelevant to the present invention as it existed in Australia before thepriority date of each claim of this application.

SUMMARY OF THE INVENTION

Throughout this specification the word “comprise”, or variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated element, integer or step, or group of elements, integers orsteps, but not the exclusion of any other element, integer or step, orgroup of elements, integers or steps.

The present invention preferably provides a very thin electrode arraythat can be inserted close to the modiolus. The electrode arraypreferably has minimal impact on the hydrodynamic behaviour of thecochlea and allows the user to gain maximum benefit from any residualhearing that they may possess.

A relatively thin array also has the advantage of having a relativelylight restoring force in its shape memory. This serves to minimise thepotential force on the modiolus from the thin spiral electrode. Byminimising the force on the modiolus, the electrode array can bepreferably formed to have a tighter curvature than that of the modiolus,without fear that the force of the electrode on the modiolus will causedamage from long term implantation. It also has the advantage ofproviding a snug fit of the electrode against the modiolus.

According to a first aspect, the present invention is an implantableelectrode array that can adopt a first configuration selected to allowthe array to be inserted into an implantee's body and at least a secondconfiguration wherein said electrode array is adapted to apply tissuestimulation, the array comprising:

-   an elongate carrier having a proximal end and a distal end, a    plurality of electrodes supported by the carrier at respective    spaced locations thereon in a region between the proximal end and    the distal end, the carrier being formed to preferentially adopt the    second configuration or another configuration different to said    first configuration; and-   an outer layer releasably connected to the elongate carrier, the    outer layer being formed so as to bias the carrier into the first    configuration prior to insertion of the array into the implantee's    body.

In a preferred embodiment, the implantable electrode array is a cochlearimplant electrode array that is adapted for insertion into the cochleaof an implantee. Preferably, the array is adapted for insertion into thescala tympani of the cochlea of the implantee.

In a further embodiment, the elongate carrier has an inner surfacedesigned to conform, following implantation, substantially with theinner wall of the cochlea.

In a still further embodiment, at least one of the electrodes has asurface that is at least adjacent the inner surface of the carrier. Morepreferably, each of the electrodes in the array have a surface that isadjacent the inner surface of the carrier. In a further embodiment, thesurfaces of the electrodes are aligned with the inner surface of thecarrier. In another embodiment, the surfaces of the electrodes standproud of the inner surface of the carrier. It is also envisaged that theelectrode surface could also be recessed into the inner surface of thecarrier. In yet another embodiment, one or more electrodes may also bepositioned on the outer surface of the carrier not facing the modiolus.Such electrodes could act as additional ground or reference electrodes.

In another embodiment, the outer layer can have a length such that itextends from the proximal end to the distal end of the carrier. Inanother embodiment, the outer layer may have a length longer than thedistance between the proximal end and distal end of the carrier. It isalso envisaged that the outer layer could have a length shorter than thedistance between the proximal end and distal end of the carrier.

In yet another embodiment, a straightening stylet may also beincorporated in the outer layer or even in the adhesive layer to ensurethat the outer layer or adhesive layer and hence the combined systemmaintains a relatively straight configuration during the insertionprocess. It is also envisaged that the outer layer could be reinforcedwith one or more metal or plastic reinforcing ribs to maintain the outerlayer and hence the electrode array in a substantially straightconfiguration.

In another embodiment, the outer layer may be moulded with apreferential curvature opposite to the preferential curvature of thecarrier rather than being merely straight. In such a configuration,following the adhesion of the two layers together, the inherentcurvature of each of the two members can act to balance each other outresulting in a substantially straight configuration.

While the array could incorporate additional layers than that definedabove, for simplicity, the invention will mainly be described below asconsisting of the carrier and the releasably connected outer layer.

In one embodiment, the outer layer can be formed from a materialdifferent to that of the carrier. In another embodiment, the carrier andouter layer can be formed from the same material. In this latter case,the carrier and outer layer can have different mechanical propertiesdespite being formed of the same material.

In one embodiment, the carrier and the outer layer can be formed from abiocompatible elastomeric material. In one embodiment, the elastomericmaterial can be a silicone rubber. In another embodiment, the elongatecarrier and/or outer layer can be formed from a biocompatiblepolyurethane. Still further, the outer layer may be formed from apolyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) or abiocompatible metal, such as platinum, iridium, stainless steel, or ashape memory metal (eg. Nitinol™), or a shape memory polymer (SMP).

In a further embodiment, the thickness of the carrier between its innersurface and outer surface can be substantially constant for at least amajority of its length from the proximal end to the distal end. Inanother embodiment, the thickness of the carrier can change, such asdecrease, from the proximal end to the distal end. In a preferredembodiment, the carrier can be relatively more resiliently flexible in alongitudinal plane and relatively less resiliently flexible in a lateralplane. In one embodiment, the carrier can have an oval-shapedcross-section and is preferably relatively more flexible in thedirection of curvature of the cochlea and relatively stiffer in otherplanes.

In a further embodiment, the carrier can have a thickness of betweenabout 0.20 mm and 0.30 mm, more preferably 0.25 mm.

In a further embodiment, the thickness of the outer layer can beconstant for at least a majority of its length. In a still furtherembodiment, the thickness of the outer layer can change, such asdecrease, from its proximal end to its distal end.

In a still further embodiment, the thickness of the outer layer can begreater than the carrier. In one embodiment, the outer layer has athickness between about 0.3 mm and 0.55 mm.

In a preferred embodiment, the carrier is preferably relativelyresiliently flexible such that, despite its propensity to normally adopta spirally curved configuration, it adopts the substantially straight orstraight configuration of the releasably connected outer layer prior toimplantation.

Preferably, the curved configuration of the array following completionof implantation is such that the carrier can fit inside the cochlea ofthe implantee with said adjacent surfaces of the electrodes beingproximal to the modiolus of the cochlea.

The outer surface of the outer layer of the array is preferably smoothto prevent any damage to the cochlea as the array is inserted into thecochlea.

The outer layer is preferably releasably connected to the carrier by anintermediate layer of a bioresorbable adhesive that softens or dissolveson exposure to a fluid. The adhesive layer preferably softens ordissolves on exposure to a saline solution or a body fluid of theimplantee, such as cochlear fluid. As the adhesive layer softens, itpreferably becomes lubricious allowing the carrier to begin to adopt itsnormal preferential curved configuration. It is also envisaged that theadhesive layer could be made from an adhesive impregnated in a fibre(eg. in much the same way as fibreglass). The fibre could be a type ofplastics material and would make the adhesive layer stiffer, which wouldultimately assist in keeping the array straight. It is also possiblethat the fibre may assist in the transportation of fluid into theadhesive layer to promote rapid dissolution of the adhesive layer.

It will be appreciated that due to the constraints imposed by thegeometry of the cochlea and/or the presence of the carrier within thecochlea, the carrier may not adopt a curved configuration identical tothat that can be adopted by the carrier prior to it being adhered to theouter layer. In one embodiment, the carrier, prior to being adhered tothe outer layer, preferably subtends an angle at least greater thanabout 450°. Following implantation, the carrier preferably subtends anangle of at least about 360°.

In a further embodiment, the bioresorbable material of the adhesivelayer is selected from the group consisting of polyacrylic acid (PAA),polyvinyl alcohol (PVA), polylactic acid (PLA) and polyglycolic acid(PGA). It is envisaged that other similar materials could also be used.

In one embodiment, at least a portion of the carrier may be releasablyattached to at least a portion of the outer layer by an attachment meansother than said adhesive layer. For example, the distal end of thecarrier may be releasably attached to the distal end of the outer layer.More than one such attachment means may exist between the carrier andthe outer layer. The attachment means may be provided by anon-bioresorbable adhesive. In another embodiment, the attachment meansmay comprise an integral connection between the carrier and outer layer.Still further, the attachment means may be a disengageable attachmentmeans.

In a preferred embodiment, the electrode array can include electricallyconducting connections, such as wires, connected to the electrodes andextending to at least said proximal end. In one embodiment, theconnections can be connected to each of said electrodes. In anotherembodiment, at least two connections can be connected to each of saidelectrodes.

Each electrode can comprise a contact element. The carrier, whenstraightened, can have a longitudinal axis with each contact elementarranged orthogonally to the longitudinal axis. The contact elements canbe formed from a biocompatible material. The biocompatible material ofthe contact element can be platinum. The wires are preferably connectedto the contact elements by welding.

Once implanted, the electrodes of the carrier member preferably receivestimulation signals from a stimulator means. The stimulator means ispreferably electrically connected to the elongate carrier member by wayof an electrical lead. The lead can include the one or more wiresextending from each electrode of the array mounted on the elongatemember.

In one embodiment, the lead can extend from the elongate member to thestimulator means or at least the housing thereof. In one embodiment, thelead is continuous with no electrical connectors, at least external thehousing of the stimulator means, required to connect the wires extendingfrom the electrodes to the stimulator means. One advantage of thisarrangement is that there is no requirement for the surgeon implantingthe device to make the necessary electrical connection between the wiresextending from the electrodes and the stimulator means.

The stimulator means is preferably positioned within a housing that isimplantable within the implantee. The housing for the stimulator meansis preferably implantable within a recess in the bone behind the earposterior to the mastoid.

When implanted, the housing preferably contains, in addition to thestimulator means, a receiver means. The receiver means is preferablyadapted to receive signals from a controller means. The controller meansis, in use, preferably mounted external to the body of the implanteesuch that the signals are transmitted transcutaneously through the skinof the implantee.

Signals can preferably travel from the controller means to the receivermeans and vice versa. The receiver means can include a receiver coiladapted to receive radio frequency (RF) signals from a correspondingtransmitter coil worn externally of the body. The radio frequencysignals can comprise frequency modulated (FM) signals. While describedas a receiver coil, the receiver coil can preferably transmit signals tothe transmitter coil which receives the signals.

The transmitter coil is preferably held in position adjacent theimplanted location of the receiver coil by way of respective attractivemagnets mounted centrally in, or at some other position relative to, thecoils.

The external controller can comprise a speech processor adapted toreceive signals output by a microphone. During use, the microphone ispreferably worn on the pinna of the implantee, however, other suitablelocations can be envisaged, such as a lapel of the implantee's clothing.The speech processor encodes the sound detected by the microphone into asequence of electrical stimuli following given algorithms, such asalgorithms already developed for cochlear implant systems. The encodedsequence is transferred to the implanted receiver/stimulator means usingthe transmitter and receiver coils. The implanted receiver/stimulatormeans demodulates the FM signals and allocates the electrical pulses tothe appropriate attached electrode by an algorithm which is consistentwith the chosen speech coding strategy.

The external controller further comprises a power supply. The powersupply can comprise one or more rechargeable batteries. The transmitterand receiver coils are used to provide power via transcutaneousinduction to the implanted receiver/stimulator means and the electrodearray.

While the implant system can rely on external componentry, in anotherembodiment, the controller means, including the microphone, speechprocessor and power supply can also be implantable. In this embodiment,the controller means can be contained within a hermetically sealedhousing or the housing used for the stimulator means.

In yet a further embodiment, a longitudinal lumen can extend through thecarrier and/or the outer layer for at least a portion of theirrespective lengths. The lumen can act as a substance delivery means fordelivering a bioactive substance to the implant site followingimplantation.

The lumen can act as a reservoir for the bioactive substance. In oneembodiment, the bio-active substance in the reservoir can leach from thelumen into the surrounding material of the carrier or outer layer andeventually migrate out of the array to the desired site of action forthe bio-active substance. In another embodiment, the carrier and/orouter layer can have one or more substance egress means whereby thebioactive substance can move out of the lumen and through the carrierand/or outer layer to a position that is preferably close to the desiredsite of action for the bio-active substance.

Where the bioactive substance is carried in or comprises a fluid, eachsubstance egress means preferably comprises a fluid egress means.

Each fluid egress means preferably has a valve means that allows fluidto exit the lumen but prevents, or at least substantially prevents,fluid flow from external the elongate member back into the lumen.

In a further embodiment, the proximal opening of the lumen can be influid communication with an additional reservoir for the bioactivesubstance that is external to the array. A pumping means, such as anosmotic pump, can transfer the bioactive substance from the additionalreservoir into the lumen for subsequent delivery to the appropriate siteof action.

It is also envisaged that the bioactive substance can be captured in theform of a solid pellet. An example of how this may occur is byimpregnating the bioactive substance in a ceramic or a polymer pelletthat has a predetermined rate of release of the bioactive substance.This solid pellet can then be stored in the lumen reservoir or in anexternal reservoir connectable to the lumen.

In one embodiment, the bioactive substance can comprise a steroid. Inanother embodiment, the bioactive substance can perform a function ofreducing the resting neuron potential of neurons within the cochlea. Theuse of such substances can result in less energy being required toexcite the neurons and cause stimulation. In yet another embodiment, thebioactive substance can comprise a nerve growth factor and mimetics toenhance the survival and growth of the nerve cells in the cochlea toimprove the performance of the cochlear implant.

In a still further embodiment, at least a portion of the surface of thecarrier can have a coating of lubricious material. In a furtherembodiment, a substantial portion of the surface can have a coating ofthe lubricious material.

The lubricious material preferably becomes lubricious on being broughtinto contact with a fluid, such as a saline solution. Still further, thecoating preferably becomes lubricious on being brought into contact witha body fluid, such as cochlear fluid.

In one embodiment, the lubricious material is selected from the groupconsisting of polyacrylic acid (PM), polyvinyl alcohol (PVA), polylacticacid (PLA) and polyglycolic acid (PGA). It is envisaged that othersimilar materials could also be used. It is envisaged that thelubricious material can also be impregnated with the bioactive substanceallowing the coating to perform a dual role. The rate of delivery of thebioactive substance can be programmed by design of the coatingstructure.

In a still further embodiment, the array can be enveloped by astiffening sheath which is made of a material that is relatively stifferthan the resiliently flexible material of the carrier. The stiffeningsheath can be adapted to assist in biasing the array into at least asubstantially straight configuration prior to implantation. In oneembodiment, the stiffening sheath can be overlaid by the coating oflubricious material.

The stiffening sheath, if present, can be formed of a bioresorbablematerial which dissolves or softens on exposure to a fluid. Thestiffening sheath can dissolve or soften on exposure to a salinesolution or a body fluid of the implantee, such as cochlear fluid, andin doing so also release one or more bio-active substances impregnatedtherein.

In a further embodiment, the bioresorbable material of the stiffeningsheath is selected from the group consisting of polyacrylic acid (PM),polyvinyl alcohol (PVA), polylactic acid (PLA) and polyglycolic acid(PGA). It is also envisaged that other suitable materials could also beused. It is envisaged that the bioresorbable element of the stiffeningsheath can also be impregnated with one or more bioactive substancesallowing the stiffening sheath to perform a dual role. The rate ofdelivery of the bioactive substance can be programmed by design of thesheath structure.

The array can still further include an additional layer surrounding thestiffening sheath. The additional layer can have a first rate of fluidingress therethrough and have at least one fluid ingress means formedtherein, the rate of fluid ingress through the fluid ingress means beinggreater than the first rate of fluid ingress through the additionallayer. In this embodiment, the coating of lubricious material can becoated on the outside of the additional layer.

The fluid ingress means can comprise one or more openings in theadditional layer. The openings can be closable. The openings cancomprise slits in the additional layer. The slits can be formed to allowsubstantially the same or the same rate of ingress of fluid through theadditional layer. In another embodiment, at least one slit can allow adifferent rate of progress of fluid through the additional layercompared to the other slits.

The present invention provides a surgeon with a cochlear implantelectrode array that can potentially be inserted to a greater depth thanhitherto known electrode arrays whilst maintaining close proximitybetween the surfaces of the electrodes and the modiolus. In particular,by removing the need to withdraw a stylet or other stiffening memberfrom the array during implantation, the surgeon is provided with a morestraightforward means of achieving successful implantation of the array.Still further, the device can be used to deliver a relatively thinelectrode array to within the cochlea. The placement of a relativelythin electrode array should help to preserve the hydrodynamic behaviourof the cochlea, thus also giving the maximum possibility of preservingresidual hearing.

According to a further aspect, the present invention is a method ofmanufacturing an implantable electrode array that can adopt a firstconfiguration selected to allow the array to be inserted into animplantee's body and at least a second configuration wherein saidelectrode array is adapted to apply tissue stimulation, the methodcomprising the steps of:

(i) moulding an elongate carrier in a curved configuration in a firstmould from a resiliently flexible material about an array of electrodessuch that at least one of the electrodes has a surface that is at leastadjacent an inner surface of the carrier;

(ii) separately moulding at least an outer layer of the array in asecond mould from a resiliently flexible material, the outer layer beingmoulded such that in combination with the elongate carrier the electrodearray is in a substantially straight or straight configuration;

(iii) removing the elongate carrier and the outer layer from therespective first and second moulds; and

(iv) straightening the carrier member and removably adhering it to theouter layer using a bioresorbable adhesive.

The carrier can be formed about an array of electrodes such that atleast one of the electrodes has a surface that is at least adjacent aninner surface of the carrier.

In a preferred embodiment, the electrodes can comprise a plurality ofcontact elements. The electrodes can be formed from a biocompatiblematerial, such as platinum. The electrode array is preferably formed bypositioning a series of platinum contact members on or about alongitudinal support that is preferably removable once the carrier hasbeen moulded about the electrodes. The support can comprise a wire thatis coated with a low-friction material, such as polytetrafluroethylene(PTFE).

Once positioned about the PTFE-coated wire, a series of electricallyconducting wires can be welded to each of the electrodes. Each electrodepreferably has at least one, and more preferably two, electricallyconducting wires welded thereto. Alternatively, the electrodes could befabricated by thin film photo-lithographic techniques as described inthe Applicant's U.S. Pat. No. 5,720,009 the contents of which areincorporated herein by reference.

Once the electrodes are positioned about the PTFE-coated wire, theelectrodes are placed in the first mould to allow moulding of thecarrier.

In a preferred embodiment, the electrode array is adapted for insertioninto the cochlea. As such, it is preferred that the electrode array,once implanted, will adopt a configuration that substantially matchesthe spiral configuration of the cochlea. To assist the electrode arrayin forming a spiral configuration following implantation, the carrier ismoulded in a spirally curved mould. The first mould preferably forms acarrier that subtends an arc of around 4500 once formed. The first mouldis preferably specifically adapted to form a carrier having a spiralshape that subtends an arc greater than that will be subtended by thearray following completion of the implantation procedure. This isachieved by designing the mould so that the radius of each turn isreduced over the length of the mould, so that a tight curvature of theouter layer is achieved.

The second mould used to mould the outer layer is preferablysubstantially straight or straight such that the outer layer, followingits manufacture and removal from the second mould, is substantiallystraight or straight, respectively.

The carrier is preferably formed from an elastomeric silicone materialand remains in the first mould at least until completion of curing. Thecured carrier may be straightened by and held straight in astraightening jig following its removal from the first mould.

The outer layer is also preferably formed from an elastomeric siliconematerial. The outer layer can be formed from the same material as thecarrier or a different material. A straightening stylet may also beincorporated in the outer layer or even in the adhesive layer to ensurethat the combined array maintains a relatively straight configurationduring the insertion process. It is also envisaged that the outer layercould be reinforced with one or more metal or plastic reinforcing ribsto maintain the system in a substantially straight configuration.

In another embodiment, the outer layer may be moulded with apreferential curvature opposite to the preferential curvature of thecarrier rather than being merely straight. In such a configuration,following the adhesion of the two layers together, the inherentcurvature of the two members acts to negate each other resulting in asubstantially straight configuration.

Once the outer layer is cured, it can be removed from the second mould.Following removal, the outer layer can then be adhered to the top of thecured carrier that is held in the straightening jig.

Once the outer layer is adhered to the carrier, the array formed by thisprocess can be removed from the jig. The adhesive used to bond thecarrier and outer layer can also help to retain the array in a straightor substantially straight configuration despite the propensity of thecarrier to want to return to its moulded spiral shape.

According to a third aspect, the present invention is a method ofinserting an implantable electrode array in a cochlea of an implanteecomprising the steps of:

(i) performing a cochleostomy;

(ii) inserting the electrode array of claim 6 through the cochleostomy;and

(iii) closing the cochleostomy.

In step (ii) of the method, the adhesive layer bonding the outer layerand carrier preferably gradually dissolves thereby allowing the carrierto move towards its second configuration.

During insertion into the cochlea, the electrode array would preferablyconform to the outer wall, or if a malleable stylet is incorporated intothe structure of the array, the array would conform to the shape of thecochlea upon insertion. The surgeon would cease insertion once theelectrode array has been inserted to an appropriate depth, which may beindicated by a depth marker on the array. As the electrode array isexposed to cochlear fluid, the adhesive bonding the outer layer andcarrier would begin to gradually dissolve. The rate of dissolution wouldbe dependent on factors such as the degree of exposure of the adhesivelayer to the cochlear fluid. Alternatively, or in addition, the surgeoncould introduce a sterile saline type solution to the array followinginsertion to aid in the dissolution process.

As the adhesive softens and becomes more lubricious, the carriergradually begins to move towards it natural curved configuration. As thecarrier curves, it separates from the outer layer.

Following insertion and at dissolution of the adhesive layer, the outerlayer can be withdrawn from the cochlea. This has the advantage ofleaving only the relatively thin carrier positioned in the scalatympani.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, a preferred embodiment of the invention is nowdescribed with reference to the accompanying drawings, in which:

FIG. 1 is a pictorial representation of a prior art cochlear implantsystem;

FIG. 2 is a simplified longitudinal sectional view of a cochlearelectrode array according to the present invention in its pre-insertionstate;

FIG. 3 is a sectional view taken along line 3-3 of FIG. 2;

FIGS. 4, 5, 6 and 8 are simplified schematic illustrations of theinitial, intermediate and final stages of the insertion of a cochlearelectrode array according to the present invention into the cochlea of ahuman being; and

FIG. 7 is a plan view of a first mould according to the presentinvention for forming a carrier as defined herein.

PREFERRED MODE OF CARRYING OUT THE INVENTION

Before describing the features of the present invention, it isappropriate to briefly describe the construction of one type of knowncochlear implant system with reference to FIG. 1.

Known cochlear implants typically consist of two main components, anexternal component including a speech processor 29, and an internalcomponent including an implanted receiver and stimulator unit 22. Theexternal component includes a microphone 27. The speech processor 29 is,in this illustration, constructed and arranged so that it can fit behindthe outer ear 11. Alternative versions may be worn on the body. Attachedto the speech processor 29 is a transmitter coil 24 that transmitselectrical signals to the implanted unit 22 via a radio frequency (RF)link.

The implanted component includes a receiver coil 23 for receiving powerand data from the transmitter coil 24. A cable 21 extends from theimplanted receiver and stimulator unit 22 to the cochlea 12 andterminates in an electrode array 20. The signals thus received areapplied by the array 20 to the basilar membrane 8 and the nerve cellswithin the cochlea 12 thereby stimulating the auditory nerve 9. Theoperation of such a device is described, for example, in U.S. Pat. No.4,532,930.

As depicted diagrammatically in FIG. 1, the cochlear implant electrodearray 20 has traditionally been inserted into the initial portion of thescala tympani of the cochlea 12 up to about a full turn within thecochlea. The electrode array according to the present invention isadapted to be inserted more deeply into the cochlea 12 than hashistorically been the case.

One embodiment of a cochlear implant electrode array, according to thepresent invention, is depicted generally as 30 in the remainingdrawings.

The array 30 comprises an elongate electrode carrier member 31 ofsubstantially oval cross-section, an intermediate adhesive layer 32, andan outer layer 33. The depicted carrier member 31 and outer layer 33 areeach moulded from a suitable biocompatible resiliently flexiblesilicone. In the depicted embodiments, each layer 31,33 is formed fromthe same elastomeric silicone material. It will be understood by aperson skilled in the art that the layers 31,33 could be formed fromdifferent materials to one another should a particular characteristic ofthe material be desired. Further, layer 33 could also include one ormore reinforcing/stiffening ribs of a suitable material such as metal orplastic. In addition to or instead of the ribs, the outer layer 33 canhave a lumen extending at least partially therethrough that is adaptedto receive a metal or plastic straightening stylet. Such a stylet can beremovable from the lumen of the outer layer 33 on insertion of theelectrode array 30 into the cochlea of the implantee.

Following its manufacture, the array 30 is maintained in a substantiallystraight or straight configuration ready for insertion in the scalatympani of a human cochlea 12.

While the depicted array 30 normally adopts a substantially straight orstraight configuration, following manufacture, it will be appreciatedthat the array 30 could be manufactured to adopt another configurationfollowing manufacture.

The carrier member 31 has a first distal end 34 that is firstly insertedinto the cochlea 12 upon insertion of the array 30. The carrier memberalso has an inner surface 35 adapted to be positioned at least close tothe surface of the modiolus of the cochlea 12 following insertion of thearray 30. Disposed within the carrier member 31 are a plurality ofelectrodes 36. It will be appreciated that the electrodes 36 depicted inthe drawings are not necessarily drawn to scale. Further, more or lesselectrodes 36 than that depicted in the drawings could be utilised inthe carrier member 31. Each electrode 36 comprises a platinum contactsurface having an outer surface at least adjacent, and preferablysubstantially aligned with, the inner surface 35 of the carrier member31. Each electrode 36 could also have a surface coating of some othermetal such as Iridium or Rhodium and they could also-be made from avariety or combination of metals. At least one electrically conductingwire 40 extends from each of the electrodes 36 through the elongatemember 31 to at least the proximal end 37 of the carrier member 31. Thewires 40 extend back to an implanted receiver/stimulator unit, such asunit 22 depicted in FIG. 1.

The depicted adhesive layer 32 is formed from a bioresorbable materialthat softens or dissolves on exposure to cochlear fluid. As the adhesivelayer 32 dissipates, it preferably becomes lubricious allowing the outerlayer 33 to begin to separate from the carrier 31 and so allow thecarrier to begin to adopt its normal preferentially curvedconfiguration. It will, however, be appreciated that due to theconstraints imposed by the geometry of the cochlea 12, the carrier 31may not adopt a curved configuration identical to that that can beadopted by the carrier 31 prior to it being adhered to the straightouter layer 33.

In one embodiment, the carrier 31, prior to being adhered to the outerlayer 33, subtends an angle at least greater than about 450°. Followingimplantation, the carrier 31 preferably subtends an angle of at leastabout 360°.

In the depicted embodiment, the bioresorbable material of the adhesivelayer 32 is selected from the group consisting of polyacrylic acid(PAA), polyvinyl alcohol (PVA), polylactic acid (PLA) and polyglycolicacid (PGA). It is envisaged that other similar materials could also beused.

In the depicted embodiment, both the carrier 31 and the outer layer 33are attached only by the adhesive layer 32. In this embodiment the outerlayer 33 becomes fully disengaged from the carrier 31 allowing the outerlayer 33 to be completely removed from the cochlea leaving only the thincarrier 31 positioned proximal the modiolus (as depicted in FIG. 8). Inanother embodiment, it will be appreciated that the distal end 34 of thecarrier 31 may be attached to the distal end 41 of the outer layer by anon-bioresorbable adhesive. A similar attachment could also be providedbetween the proximal end 43 of the outer layer 33 and the carrier 31. Itcould be possible to have more than two such attachments between thecarrier 31 and the outer layer 33. In such an embodiment, the outerlayer would lengthen allowing the carrier to assume its positionproximal to the modiolus. This could be done in a number of ways such asconstructing the outer layer with a hydrogel type of material wherebyupon exposure to fluid the outer layer would swell in a lengthwise mode.The outer layer could also achieve this by being constructed in aconcertina-type arrangement and assembled in its compact form throughuse of a bioresorbable adhesive. In this arrangement, upon exposure tofluid, the outer layer would be permitted to expand hence allowing thecarrier to assume its position proximal to the modiolus.

Prior to implantation, the outer layer 33 has a sufficient stiffness toretain the silicone elongate carrier member 31 in a straightconfiguration. As previously stated, it is also considered permissiblethat the outer layer could itself be pre-curved in an opposite manner tothe pre-curve of the carrier so that together the array configurationwould be naturally balanced straight. The bioresorbable material of theadhesive layer 32 can also be used to assist in retaining the straightconfiguration of the array 30.

The array 30 is typically delivered to a surgeon in a sterile package.Upon removal from the package, the array 30 is inserted into the scalatympani of the cochlea 12 as is depicted in FIG. 4, with the innersurface 35 of the carrier 31 directed towards the inner wall 44 of thecochlea 12. The array 30 is advanced into the cochlea by the surgeonpushing the array in direction of arrow A.

As the pushing continues, the array 30 enters the spirally curvedsection of the cochlea 12, with the outer surface of the outer layer 33gliding gently along the radially outer wall 45 of the cochlea 12 (FIG.5). As insertion continues and dissolution of the adhesive layer 32continues, the carrier 31 begins to become detached from the outer layer33 and the natural preferential curvature of the carrier 31 serves tomove the carrier member 31 towards the inner wall 44 of the cochlea 12,with the result that the electrodes 36 facing the modiolus within thecochlea 12 are positioned as close as possible to the spiral ganglia 46thereof (see FIG. 6).

The design of the cochlear implant electrode array 30 allows theelongate carrier member 31 to be inserted, in a typical case, moredeeply into the scala tympani of the cochlea 12 than would be the casefor hitherto traditionally used perimodular arrays, such as array 20depicted in FIG. 1.

Following detachment of the carrier 31 from the outer layer 33 as isshown in FIG. 6, the outer layer 33 is withdrawn from the cochlea 12leaving in place a thin electrode array proximal to the modiolus (FIG.8).

To form the depicted electrode array 30, a plurality of platinum contactmembers, which become the electrodes 36, are mounted on a PTFE-coatedwire. Each electrode 36 could also have a surface coating of some othermetal such as Iridium or Rhodium and they could also be made from avariety or combination of such suitable metals. Each contact member 36has one, or possibly two, conductive wires welded thereto to allowelectrical connection from the implanted receiver/stimulator unit 22 tothe contact members 36.

Once formed, the electrode assembly is placed in a first spirally curvedmould (such as the spiral-shaped channel 51 of second mould 50 depictedin FIG. 7), with the outer surfaces of the contact members 36 abuttingthe inner surface thereof. Once correctly positioned in the first mould50, a silicone is poured or injected into the first mould 50 around thePTFE-coated wire and allowed to cure.

Meanwhile, the outer layer is formed in a second straight mould or ismoulded in a mould so as to have a preferential curvature that is in adirection opposite to that of the carrier. Once cured, the formedelastomeric outer layer 33 can be removed from the second mould. Theouter layer 33, on removal from the second mould, naturally adopts thestraight configuration of the second mould.

Once removed from the first mould 50, the carrier 31 is straightened ina straightening jig. The straightening jig can comprise two grips thatgrip the respective ends of the carrier 31 and tension the carrier 31 toensure it is at least substantially straight. The straight outer layer33 is then adhered, using adhesive layer 32, to the carrier member 31.

Once the outer layer 33 has adhered to the carrier member 31, the array30, the PTFE-coated wire can be removed from the member 31. The removalof the PTFE-coated wire leaves a longitudinal lumen in the carrier 31(not depicted).

The use of the process as defined herein results in the formation of astraight electrode array 30 that will begin to adopt a curvedconfiguration following implantation.

While the preferred embodiment of the invention has been described inconjunction with a cochlear implant, it is to be understood that thepresent invention has wider application to other implantable electrodes,such as electrodes used with pacemakers.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

1. An implantable electrode array with an elongate carrier, that canadopt a first substantially straight configuration selected to allow thearray to be inserted into an implantee's ear and a preferential secondspirally curved configuration wherein said electrode array is adapted toapply tissue stimulation, the array comprising: a proximal end; a distalend; a plurality of electrodes supported by the elongate carrier atrespective spaced locations thereon in a region between the proximal endand the distal end; and a non-bioresorbable outer layer having a length,which biases the elongate carrier into said first substantially straightconfiguration, the outer layer being removably adhered to the elongatecarrier along a majority of the length of the outer layer until fullinsertion of the array into the implantee's ear, and removable afterfull insertion to allow the elongate carrier to adopt the preferentialsecond spirally curved configuration.
 2. An implantable electrode arrayof claim 1 wherein the implantable electrode array is a cochlear implantelectrode array insertable into the cochlea of an implantee.
 3. Animplantable electrode array of claim 2 wherein the elongate carrier hasan inner surface that is substantially conformable, following insertion,with an inner wall of the cochlea.
 4. An implantable electrode array ofclaim 3 wherein each of the electrodes have a surface that is at leastadjacent the inner surface of the carrier.
 5. An implantable electrodearray of claim 1 wherein the outer layer has a length that extends fromthe proximal end to the distal end of the carrier.
 6. An implantableelectrode array of claim 5 wherein a straightening stylet extendsthrough the outer layer to at least substantially straighten the outerlayer and hence the electrode array into the first substantiallystraight configuration.
 7. An implantable electrode array of claim 2wherein the outer layer is reinforced with one or more reinforcing ribsto maintain the outer layer and hence the electrode array in the firstsubstantially straight configuration.
 8. An implantable electrode arrayof claim 2 wherein the outer layer is moulded with a preferentialcurvature opposite to the preferential curvature of the carrier therebyresulting in the electrode array having the first substantially straightconfiguration.
 9. An implantable electrode array of claim 2 wherein thecarrier and the outer layer are each formed from a biocompatibleelastomeric material.
 10. An implantable electrode array of claim 2wherein the outer layer is removably adhered to the carrier by a layerof bioresorbable adhesive.
 11. An implantable electrode array of claim10 wherein the adhesive layer is formed from a bioresorbable materialthat softens or dissolves on exposure to a body fluid of the implantee.12. An implantable electrode array of claim 11 wherein, followinginsertion, the carrier subtends an angle of at least about 360°.
 13. Animplantable electrode array of claim 11 wherein the bioresorbablematerial of the adhesive layer is selected from the group consisting ofpolyacrylic acid (PAA), polyvinyl alcohol (PVA), polylactic acid (PLA),and polyglycolic acid (PGA).
 14. A method of inserting an implantableelectrode array in a cochlea of an implantee comprising the steps of:(i) performing a cochleostomy; (ii) fully inserting the electrode arrayof claim 2 through the cochleostomy; (iii) removing thenon-bioresorbable outer layer from the elongate carrier after fullinsertion; and (iv) closing the cochleostomy.
 15. A method of insertingan implantable electrode array in a cochlea of an implantee of claim 14wherein during step (ii) the adhesive layer bonding the outer layer andcarrier gradually dissolves thereby allowing the carrier to move towardsits second configuration.
 16. A method of inserting an implantableelectrode array in a cochlea of an implantee of claim 14 whereinfollowing insertion of the electrode array, and at least partialdissolution of the adhesive layer, the outer layer is withdrawn from thecochlea.
 17. A method of forming an implantable electrode array that canadopt a first substantially straight configuration selected to allow thearray to be inserted into an implantee's body and a preferential secondspirally curved configuration wherein said electrode array is adapted toapply tissue stimulation, the method comprising the steps of: mouldingan elongate carrier in a curved configuration in a first mould from aresiliently flexible material about an array of electrodes such that atleast one of the electrodes has a surface that is at least adjacent aninner surface of the carrier; separately moulding a non-bioresorbableouter layer of the array in a second mould from a resiliently flexiblematerial, the outer layer being moulded in a substantially straight orstraight configuration; removing the elongate carrier and the outerlayer from the respective first and second moulds; and straightening thecarrier member and removably adhering it to the outer layer along amajority of the length of the outer layer using a bioresorbableadhesive.
 18. A method of forming an implantable electrode array ofclaim 17 wherein the electrode array is insertable into the cochlea ofan implantee.
 19. A method of forming an implantable electrode array ofclaim 17 wherein the first mould used to mould the carrier isspiral-shaped such that the carrier, following its manufacture andremoval from the first mould, subtends an arc of about 450° or greater.20. A method of forming an implantable electrode array of claim 17wherein the second mould used to mould the outer layer is substantiallystraight or straight such that the outer layer, following itsmanufacture and removal from the second mould, is substantially straightor straight, respectively.
 21. A method of forming an implantableelectrode array of claim 20 wherein once the carrier is cured, thecarrier is removed from the first mould and straightened in astraightening jig.
 22. A method of forming an implantable electrodearray of claim 21 wherein once the outer layer is cured, the outer layeris removed from the second mould and adhered to the carrier being heldin the straightening jig.
 23. A method of forming an implantableelectrode array of claim 22 wherein once the outer layer is adhered tothe carrier, the electrode array formed by this process is removed fromthe jig and packaged.